# Electric Drives Questions and Answers – Motors Rating and Heating – Environmental Factors

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This set of Electric Drives Multiple Choice Questions & Answers (MCQs) focuses on “Motors Rating and Heating – Environmental Factors “.

1. Which test is used to determine the conditions where electric motors are being used?
a) Thermography
b) Geography
c) Seismography
d) Anthropology

Explanation: Thermography is used to detect heat patterns. It uses an Infrared rays camera. This test is used to determine the conditions where electric motors are being used.

2. What is the range of size of the motor (kW) for efficiency nearly equals to 90%?
a) 15-150
b) 0-2
c) 3-15
d) 150-250

Explanation: As the size of the motor increases its speed decreases. The range of size of the motor (kW) for efficiency nearly equals to 90 % is 15-150. The efficiency increases as the size increases.

3. Calculate the heat loss in the electric motor for time interval 0-5 sec using the following data: Ia=2 A, Ra=.8Ω.
a) 16 J-sec
b) 14 J-sec
c) 12 J-sec
d) 10 J-sec

Explanation: The heat loss can be calculated using the Joule law of heating effect. The value of the heat loss is Ia2×Ra×t=4×4=16 J-sec.
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4. 1 Btu/h is equal to ___________
a) .641
b) .334
c) .293
d) .417

Explanation: One British thermal unit is equal to .293 Watt. 1 Btu/h is the amount of energy requires to increase the temperature of one pound of water by 1℉.

5. Overloading can be prevented using _____________
a) Over-current protection
b) Speed protection
c) Over frequency protection
d) Oversize protection

Explanation: Overloading can be prevented using over-current protection. It will detect the over-current and interrupt the supply. IDMT over-current relay can be used for the protection purpose.
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6. Full form of IDMT is _________
a) Inverse definite minimum time
b) Inverter definite minimum time
c) Inverter definite maximum time
d) Insert definite minimum time

Explanation: IDMT stands for Inverse definite minimum time over-current relay. It has both the characteristics of inverse over-current and definite time over-current relay.

7. The most common cause of motor failure is ____________
b) Low resistance
c) Contamination
d) Over-Heating

Explanation: The most common cause of motor failure is low resistance. Overheating, corrosion can damage the insulation of the windings which leads to low resistance.

8. Full form of THD is ____________
a) Total harmonic diverter
b) Total harmonic digital
c) Total harmonic distortion
d) Total harmonic discrete

Explanation: THD stands for total harmonic distortion. It measures the harmonic distortion in the signal. According to IEEE-1992, only 5% of THD is valid.

9. Calculate the compensator rating required for a sec(Φ)=1.
a) 0.64 P.U
b) 0.21 P.U
c) 0.57 P.U
d) 0 P.U

Explanation: The compensator rating can be calculated using the relation QP.U=√1-cos2(∅)=0. This per unit value VAR compensator is required to improve the power factor of the system.

10. Calculate the value of Voltage ripple factor if the Form factor is 1.1.
a) 45.8 %
b) 46.2 %
c) 47.5 %
d) 49.9 %

Explanation: Voltage ripple factor measures the harmonics on the DC side of the converter. The value of voltage ripple factor is √F.F2-1=45.8 %. The form factor is Vr.m.s÷Vavg.

11. Calculate the value of THD if the distortion factor is .8.
a) 76 %
b) 75 %
c) 74 %
d) 73 %

Explanation: THD measures the harmonics distortion in the signal. The value of THD is √(1÷D)2-1=75 %. The value of the distortion factor is Ior.m.s÷IR.m.s.

12. Full form of VRF.
a) Voltage ripple factor
b) Voltage revert factor
c) Volume ripple factor
d) Volume revert factor

Explanation: Voltage ripple factor measures the harmonics on the DC side of the converter. The value of voltage ripple factor is √F.F2-1. The form factor is Vr.m.s÷Vavg.

13. Second lowest order harmonic present in 3-Φ fully controlled rectifier is __________
a) 5th
b) 7th
c) 3rd
d) 2nd

Explanation: Only 6k±1 order harmonics are present in 3-Φ fully controlled rectifier. The second lowest order harmonic is 7th harmonic for k=1. Third order harmonics are absent in 3-Φ fully controlled rectifier.

14. Electric motors are placed in __________ to protect them from dust and contamination.
a) Enclosures
b) Open environment
c) Moisturizing environment
d) Hot environment

Explanation: Electric motors are placed in enclosures to protect them from dust and contamination. There are seven most common types of enclosures. For example- Open Drip Proof.

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